This invention relates to remote power controllers which are used to control loads in a power distribution system, and more particularly, to such controllers which limit the current to a specific level in the case of an overload.
Remote power controllers have used bipolar transistors to limit fault currents in circuits of power distribution systems. One current limiting technique limits the base current to a level which allows the transistor to pull out of saturation when the collector current reaches a level proportional to the product of the gain of the transistor multiplied by the base current. The gain of a bipolar transistor varies considerably over temperature and from device to device, thereby requiring sensing circuits to adjust for different conditions. An alternative technique senses the load current and adjusts the base current of the transistor accordingly. These techniques have the drawback that bipolar transistors will not limit current instantaneously and current surges well beyond the current limit level can occur. Typically, the worst response occurs when a short circuit is applied while full rated load current is flowing.
Some remote power controllers depend upon the inductance of the transmission line in the distribution system to limit the rise time of the current. However, low values of transmission line inductance lead to high peak currents which occur before the bipolar transistors and control circuitry can react to limit the current. Since transmission line inductance is indeterminate and could be almost zero in certain distribution systems, this is not an effective method of limiting current.
In order to take advantage of the relatively high gain, good temperature stability, high input impedance and freedom from second breakdown exhibited in more recently developed field effect transistors, a DC power controller which uses an FET as the switching element was designed and is disclosed in U.S. Pat. No. 4,404,473, issued Sept. 13, 1983. Overcurrent protection in that power controller was achieved by turning off the FET gate voltage signal in response to a predetermined magnitude of load current. A time delay was introduced between the time that the predetermined magnitude of load current was reached and the time that gate drive signal was removed to turn off the FET. The gate of the FET is driven with a voltage such that the device will not limit current to a level below its rated maximum value. This permits a current surge prior to turn-off of the FET. The resulting current surge, occurring between the time that the fault occurs and the time that the control circuit removes the gate voltage, may destroy the FET.
It is therefore desirable to construct a remote power controller which limits fault currents to a specific maximum magnitude and is not subject to the fault current surges experienced in prior art power controllers.